Protection circuit, interface circuit, and communication system

An interface circuit comprising:

• at least one of a transmission circuit capable of outputting a signal to an external terminal and a reception circuit capable of receiving a signal from the external terminal;
• and
• a protection circuit coupled to the external terminal, wherein the protection circuit includes:
• a control circuit coupled to a first power-supply wire applied with a first power-supply voltage, wherein the control circuit generates a control voltage in accordance with the first power-supply voltage and an input voltage, and
• a voltage limitation circuit coupled between a node applied with the input voltage and a second power-supply wire applied with a second power-supply voltage, wherein the voltage limitation circuit includes a variable resistance unit having a resistance value that changes according to the control voltage, and
• wherein when the first power-supply voltage is not supplied to the protection circuit and the input voltage is larger than a first voltage, the control circuit generates the control voltage such that the resistance value of the variable resistance unit is smaller than that in a case where the input voltage is equal to or less than the first voltage.

A communication system comprising:

• a first communication device;
• and
• a second communication device that is capable of communicating with the first communication device, wherein:
• the first communication device includes a first external terminal and a first interface circuit coupled to the first external terminal,
• the first interface circuit includes a resistor that pulls up the first external terminal to a first power-supply voltage,
• the second communication device includes a second external terminal and a second interface circuit coupled to the second external terminal,
• the second interface circuit is capable of being coupled to the first interface circuit via the first and second external terminals and is capable of operating under a second power-supply voltage lower than the first power-supply voltage,
• the second interface circuit includes a protection circuit coupled to the second external terminal, and
• the protection circuit includes:
• a control circuit coupled to a first power-supply wire applied with the second power-supply voltage, wherein the control circuit generates a control voltage in accordance with the second power-supply voltage and an input voltage supplied via the second external terminal, and
• a voltage limitation circuit coupled between a node applied with the input voltage and a second power-supply wire applied with a third power-supply voltage, wherein the voltage limitation circuit includes a variable resistance unit having a resistance value that changes according to the control voltage,
• wherein when the second power-supply voltage is not supplied to the protection circuit and the input voltage is larger than a first voltage, the control circuit generates the control voltage such that the resistance value of the variable resistance unit is smaller than that in a case where the input voltage is equal to or less than the first voltage.